Human immunodeficiency virus type 1 (HIV-1) is a retrovirus that is the causative agent of human acquired immunodeficiency syndrome (AIDS). In retroviruses tRNA molecules are recruited as primers to initiate reverse transcription. In the case of HIV-1 a specific host cell tRNA, human tRNALys3, serves as the initiation primer. All three human tRNALys isoacceptors are selectively packaged into the HIV-1 virion. It has recently been shown that the tRNALys binding protein, human lysyl-tRNA synthetase (LysRS) is also packaged into HIV-1 virions and into viral-like particles formed from the HIV Gag protein alone. These data and recent pull-down assays support an interaction between human LysRS and Gag in vivo and in vitro. This proposal will explore this hypothesis further by quantitatively measuring and mapping the interaction in vitro. The goals will be accomplished by employing isothermal titration calorimetry (ITC), fluorescence polarization (FP), and protein footprinting techniques. Information gained from the proposed research will provide valuable insights into the molecular interaction between LysRS and Gag. Additionally, this information may be useful in the design of new therapeutic agents against AIDS.